Gloxy is a type of coal which has an EPR signal which is very sensitive to changes of the partial pressure of oxygen (pO2). It has the potential to be one of the principal paramagnetic materials that will be used for the widening field of EPR oximetry in living subjects. An assessment of the responses of tissues to the presence of this material is essential for its effective use. The aim of this study is to ascertain histological evidence of biological interaction of the tissues to the presence of the implanted gloxy and long-term stability in tissues of the sensitivity to oxygen. Gloxy was implanted into the brain, kidney, liver, and quadriceps muscle of mice. After implantation, the tissues containing the gloxy were harvested at specific endpoints and histological examination was carried out by light and electron microscopy. Periodically EPR spectra were obtained from the live animals. Histological assessments demonstrated that gloxy is quite stable and did not induce significant reactions in the tissues examined in this study during the time period of observation. At 24 hr. some of the samples did have evidence of hemorrhage which appeared to be associated with the physical trauma of implantation and some inflammatory cells also were present. Seven days after the implantation, there was no indication of hemorrhage and at most, a small amount of inflammatory cells was observed. Throughout the observation period there was no evidence of significant inflammation or foreign tissue reaction at the site of the gloxy. Both forms of gloxy remained their sensitivity to oxygen at least throughout the experiment ( 6 months) in all of the various organs ( brain, kidney, liver, muscle).